Changed hard drive, why did my external monitor start working?

The hard drive on my wife's Mac Mini failed about a month ago. The (HP) monitor connected to that computer had, for at least a year, possibly longer, been a bit wonky. There was a steady white line near the top of the screen and, intermittently, after 30-60 minutes of usage, the screen would go blank or be otherwise partially unusable without resetting it. I'd planned on replacing that monitor once I changed out the hard drive. However, since I changed out the hard drive and installed a new OS on the computer yesterday, the monitor has been working perfectly. No white line, no blanking out, no problems otherwise.

Why would this be the case? Could it be that I inadvertently fixed an internal connection issue when I dissembled and reassembled the Mini? Or did the failing HD create the monitor problem?